1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile machine, or multifunctional apparatus combining two or more of the above functions, and in particular to an image forming apparatus employing a special color of toner.
2. Description of the Related Art
In image forming apparatuses such as copiers, printers, or facsimile machines, there is a conventionally known transfer technology in which special toner such as a transparent toner or white toner is used together with a plurality of color toners, and toner images formed of the plurality of color toners and the special color toner are superimposedly transferred to, for example, an intermediate transfer belt.
The image forming apparatus in which the toner image is formed using such special toner is configured to enable a user to switch between a normal full-color image mode, monochrome or black and white image mode, and special color or five-color image mode depending on the type of the original document or image data.
More specifically, when forming a color image or color image data of standard four colors (YMCK), a normal color image mode is selected in which a toner image of black toner and color toner images of yellow, magenta, and cyan are superimposed on the intermediate transfer belt, the transfer target member. When forming a monochrome image or image data, a monochrome image mode is selected in which toner images of black toner only are superimposed on the intermediate transfer belt. Further, when forming a color image including a special color (or color image data including a special color), a toner image of black toner, color toner images of yellow, magenta, and cyan, and a toner image of particular color are superimposed on the intermediate transfer belt.
In addition, when switching between various modes, a photoreceptor drum which is not used for image formation is separated from the intermediate transfer belt on which the special toner image is to be formed.
To be more specific, when the normal color image mode is selected, the photoreceptor drum on which the special color toner image is to be formed is separated from the intermediate transfer belt, and the photoreceptor drum on which a black toner image is formed and the three photoreceptor drums each on which a toner image of one of three colors (YMC) is formed come in contact with the intermediate transfer belt. In addition, when the monochrome image mode is selected, the photoreceptor drum to form a particular toner image and the three photoreceptor drums on each of which a toner image of one of three colors (YMC) is formed are separated from the intermediate transfer belt, and only the photoreceptor drum on which a black toner image is formed comes in contact with the intermediate transfer drum. Further, when a particular color image mode is selected, all of the photoreceptor drums on which a particular toner image is to be formed, the photoreceptor drum on which a black toner image is formed, and the three photoreceptor drums each on which a toner image of each of three colors (YMC) is formed come in contact with the intermediate transfer belt.
When, in the conventional image forming apparatuses, an operation to print various, mixed original images or image data is performed in one job continuously, switching between the normal color image mode, monochrome image mode, and special color image mode requires an operation to separate the photoreceptor drum (image carrier) from the intermediate transfer belt (transfer target member) while a continuous print job is being performed. Such a separation operation saves wear and tear on the imaging section which is not used in the image formation, but with the disadvantage that productivity of the image formation in the continuous print operation is degraded.
As a remedy for the above problem, images can be formed during separation of the photoreceptor drum or after completion of separation. However, since image formation is performed by the switched-over mode in a state in which the intermediate transfer belt is still vibrating due to the contacting and separating movement, abnormal images including color shift or uneven pitch may be formed on the output image.